Diminishing anisotropy of PrFeAsO$_{1-y}$ single crystal as temperature goes zero*

The magnetic torque of a high-quality PrFeAsO$_{1-y}$ single crystal has been investigated at temperatures from 10 K to 45 K in magnetic fields from 5 kG and 50 kG. The torque curves of the PrFeAsO$_{1-y}$ single crystal have been measured systematically. The superconducting anisotropy of PrFeAsO$_{1-y}$ is determined by analyzing the torque curves. The temperature and field dependence of $\gamma$ reflect that the Fe-oxypnictide superconductivity has the rather three- dimensional isotropic nature of Fe-As layer network. We consider that this is good evidence for the multi-band superconductivity in Fe-oxypnictide. We find that the superconducting anisotropy $\gamma$ in PrFeAsO$_{1-y}$ can be approximated by $\gamma = 1.08 + 0.0068 T$ in all fields by employing the Kogan model. We find it surprising to have such a low anisotropy parameter $\gamma$ even for a layered superconductor. Such a small anisotropic parameter in Fe-pnictides is preferential to ensure the intergrain connectivity and the resulting high critical current density $J_c$.